This application claims the priority of Japanese Patent Application No. 6-37020 filed on Mar. 8, 1994 and Japanese Patent Application Reference No. NA 95-1027 filed on Feb. 10, 1995, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium secondary battery comprising a negative electrode having lithium as an active material, a positive electrode comprising a boron-containing lithium-manganese composite oxide as an active material and a nonaqueous electrolyte comprising a solute and a solvent. More specifically, the present invention relates to an improved positive electrode active material and an improved nonaqueous electrolyte, for the purpose of providing a highly reliable lithium secondary battery with which the internal resistance minimally increases even when, due to an unpredictable accident or the like causes, it has been continuously charged at a high charge voltage for a long period of time.
2. Description of the Prior Art
Manganese dioxide, which can electrochemically absorb and discharge lithium and produce a high discharge voltage (vs. Li/Li.sup.+) has been studied as a positive electrode active material of lithium secondary batteries.
However, to make manganese dioxide usable in practice as a positive electrode active material, it is necessary to improve the stability of its crystal structure (charge-discharge cycle characteristic) during charge-discharge cycles. This is because the crystal structure of manganese dioxide, when subjected to repeated swelling and shrinkage during charge-discharge cycles, readily breaks in a short period of the cycles. To improve the poor stability of crystal structure of manganese dioxide during charge-discharge cycles, for example Japanese Patent Application Laid-open No. 114064/1988 discloses a lithium-manganese composite oxide comprising manganese dioxide and Li.sub.2 MnO.sub.3. Also, Japanese Patent Application Laid-open No. 235158/1989 discloses a lithium-containing manganese dioxide composite oxide comprising manganese dioxide containing lithium in the crystal lattice (this composite oxide is one of lithium-manganese composite oxides, too).
These lithium-manganese composite oxides, having an excellent charge-discharge characteristic, are sufficiently usable as positive electrode active materials of secondary batteries.
However, lithium secondary batteries with this type of lithium-manganese composite oxide have the following problem. That is, when they are, by mistake, subjected to a continuing application of a high voltage (at least 4 V) exceeding the usual charge voltage (about 3.5 V) for a long time, the lithium-manganese composite oxide decomposes and the decomposition products dissolve out into the nonaqueous electrolyte used, thereby markedly increasing the internal resistance of the batteries.